Because node + npm have much broader appeal and libraries already available, and those that Apple is offering could very easily fit into this system. For that matter, Apple could have pre-published them as npm modules to the local global store for their platform.

node/npm exists and is very vibrant. Embracing it would have been a better move imho. Much the same reason they didn't creat ATML (Apple Text Markup Language) as a system for documents that only macs can use. They probably would have been better served by embracing a JS platform that already works well on Macs with libraries/modules that would have worked well for their platform.